The polybutadiene formulations used as the base rubber in golf balls have hitherto been modified and improved in various ways to confer the golf balls with outstanding rebound characteristics.
For example, JP-A 62-89750 discloses rubber compositions for solid golf balls which are obtained by formulating as the base rubber a polybutadiene having a Mooney viscosity of 70 to 100 and synthesized using a nickel or cobalt catalyst, in combination with either a polybutadiene having a Mooney viscosity of 30 to 90 and synthesized using a lanthanide series catalyst or a polybutadiene having a Mooney viscosity of 20 to 50 and synthesized using a nickel or cobalt catalyst.
JP-A 2-268778 describes golf balls obtained by compounding a polybutadiene having a Mooney viscosity of less than 50 and synthesized using a Group VIII catalyst with a polybutadiene having a Mooney viscosity of less than 50 and synthesized using a lanthanide catalyst.
In addition, JP-A 11-70187 discloses multi-piece solid golf balls in which the mantle is made of a polybutadiene having a low Mooney viscosity. JP-A 11-319148 teaches solid golf balls obtained using a rubber composition formulated from a polybutadiene having a Mooney viscosity of 50 to 69 and synthesized using a nickel or cobalt catalyst in combination with a polybutadiene having a Mooney viscosity of 20 to 90 and synthesized using a lanthanide series catalyst. JP-A 11-164912 describes solid golf balls obtained using a rubber composition having a 1,2-vinyl unit content of not more than 2.0% and having a ratio Mw/Mn of the weight-average molecular weight to the number-average molecular weight of not more than 3.5. JP-A 63-275356 discloses golf balls made with a rubber composition formulated using a high Mooney viscosity polybutadiene. JP-A 3-151985 describes golf balls made with a rubber composition formulated using a polybutadiene having a high number-average molecular weight in combination with a polybutadiene having a low number-average molecular weight.
However, the golf balls in all of these prior-art disclosures have inadequate rebound characteristics.
For the crosslinking of rubber component, JP-A 61-71070 mentions the use of two types of organic peroxides and JP-A 62-112574 mentions the use of a small amount of organic peroxide. Yet, the golf balls using crosslinked rubber products obtained in both of these disclosures have inadequate rebound characteristics. Moreover, crosslinking takes a long time, leading to a lowering of productivity.
JP-A 58-225138, JP-A 2001-149505, JP-A 2001-149506 and JP-A 2001-149507 disclose rubber compositions, which still have inadequate rebound characteristics.